This invention belongs to the art of brickmaking and pertains more specifically to a method of, and apparatus for, pressure molding refractory brick commonly termed firebrick. Still more specifically, the invention is directed to such a method and apparatus whereby a mixture of desired firebrick ingredients in a mold is subjected to both pressure and vibrations for more efficient fabrication of high quality firebrick, particularly of that containing graphite, than heretofore.
The pressure molding of firebrick by presses such as a friction press and hydraulic press has been known and practiced in the brickmaking industry. With the recent advent of graphite-containing firebrick, however, the conventional brickmaking equipment has proved to be in need of improvement for higher production. Standard methods of molding graphite-containing firebrick have been either to increase the molding pressure to 1.0 to 3.0 tons per square centimeter, as compared with 0.5 to 1.0 ton per square centimeter required for the fabrication of common firebrick, or to repeat the application of pressure a number of times by a friction press.
Generally, for the pressure molding of firebrick, there is first prepared a mixture of refractory aggregate in the form of both coarse and fine particles and a binder with air entrapped in the mixture. Placed in a mold, the mixture is pressed to cause the coarse and fine aggregate particles to be bound to one another in practically the most closely packed state. High bulk density (metric units) or bulk specific gravity and low porosity are the requisites of high quality firebrick. Conventional measures for the attainment of these properties have been to increase the packed density of the mixture of brick ingredients within a metal mold at the time of pressing and forming thereby to obtain uniformity and to carry out deaeration by degassing.
However, such conventional measures have proved unsatisfactory in the case of graphite-containing firebrick. The mixture to be processed into this class of firebrick contains from 15 to 20 percent by weight of graphite as particles of uniform and balanced distribution. When the mixture is pressed, these graphite particles exhibit a unique behavior not found in the fabrication of common firebrick. Not only are the fine graphite particles themselves elastic, but they also possess even greater elasticity as a mass. This is because of a large volume of air trapped in the interstices of the graphite particles and of their great surface energies. Having a low coefficient of friction, moreover, the graphite particles very easily slip relative to each other. For these reasons the graphite-containing mixture subjected to pressing tends to regain its initial state as an elastic body.
Thus, for a higher bulk specific gravity of graphite-containing firebrick fresh from the press, it is essential to reduce the elastic properties of the firebrick mixture before it is pressed. (It is to be noted that the term "bulk specific gravity" as used in this specification means that of the green brick that has just been pressed.) The brickmaking industry has recently expended much research effort for the solution of this problem.
We have conducted a series of exhaustive experiments and have amassed data concerning how the bulk specific gravity of a graphite-containing firebrick is affected by the compositions and particle sizes of the mixed raw materials, the molding pressures, and the number of bumping impacts or pressures exerted on the mixtures. The data indicate that the repeated exertion of impacts or pressures serves to improve the bulk specific gravity, but up to a limit of approximately 20 times from the standpoint of production engineering. Also, the higher the molding pressure, the greater is the bulk specific gravity, and this tendency becomes even more pronounced in cases where the mixtures of raw materials contain a large proportion of fine particles. At the same molding pressure, however, the bulk specific gravity decreases in inverse proportion with the percentage of fine particles contained; that is, a higher molding pressure is required for the same bulk specific gravity.
Both friction press and hydraulic press have their own restrictions and limitations as heretofore used for pressure molding firebrick. The friction press is an inertia operated machine, translating the rotation of a flywheel into linear motion of a screw shaft. It has generally been employed for applying a series of impact forces or blows on the mixture in a mold. Problems arise, however, in increasing the size of the friction press to an extent required for the exertion of sufficiently high molding pressures for the fabrication of graphite-containing firebrick. The operating principle of the friction press unavoidably gives rise to considerable energy losses. No negligible proportion of the mechanical energy created by the rotation of the flywheel is wasted in the form of the heat of friction between the screw shaft and the mating part and of vibration and noise upon application of blows. Moreover, as the bulk specific gravity of the mixture being pressed rises close to the limit, the impact of each blow is transmitted amost directly to the machine itself, possibly resulting in its damage. Any attempt to increase the size of the friction press to an extent necessary for the production of graphite-containing firebrick is, therefore, impractical.
The hydraulic press, on the other hand, exerts semistatic pressures and operates with little noise and little energy loss. The manufacture of large size hydraulic presses is also relatively easy. The problem is that graphite-containing firebrick of truly satisfactory physical properties is not obtainable no matter how high the semistatic pressures of the hydraulic press are made.
Accordingly, a method known as "bumping", which comprises the repeated (approximately 20 times) application of impact vibration caused by the maximum pressure of the hydraulic press on the mixture in a mold is being used. For the application of such repeated blows, a solenoid actuated directional control valve has been employed for alternately directing hydraulic oil under pressure into the pair of opposed fluid chambers of the press and hence for causing the repeated up-and-down motion of the ram.
An objection to the known bumping method is the prolonged length of time necessary for pressing each brick. Each blow has ordinarily required a period of five to six seconds, so that a total of as much as 100 to 120 seconds has been necessary for imparting 20 blows to each brick.